1. Field of the Invention
The present invention relates to a multilayered printed circuit board having excellent heat resistance and to a resin composition for copper-clad laminates, a resin-coated copper foil, and a multilayered copper-clad laminate each suitable for use in producing the circuit board.
2. Prior Art
Laminates for use in producing printed circuit boards in the electronic industry are frequently produced by impregnating a glass cloth, craft paper, nonwoven glass fabric, or the like with a thermosetting resin such as a phenolic or epoxy resin, semicuring the resin to obtain a prepreg, and laminating the prepreg on one or each side to a copper foil. Furthermore, a multilayered printed circuit board is also produced by forming a circuit on each side of the above copper-clad laminate to obtain an inner-layer material and cladding the same on each side with a copper foil by interposing a prepreg therebetween.
With the trend toward the high densification of printed circuit boards, the formation of so-called via holes, which are minute holes not extending through the board thickness, has recently become very common. Such via holes are formed by means of a laser beam or plasma processing. Since use of a prepreg containing an inorganic ingredient, such as glass fibers, as an insulating layer results in poor processability in processing with a laser beam or plasma, resins containing no inorganic ingredients are frequently used as insulating layers. In this case, there are three methods for forming a resin layer using resins containing no inorganic ingredients, namely, the resin layer is formed by (1) directly applying a liquid resin to an inner circuit, (2) a resin film made of a semicured thermosetting resin, or (3) applying a resin to one side of a copper foil and semicuring the resin. The thus formed resin layer is laminated to a printed circuit board having a circuit (an inner-layer material), before the outer-layer copper foil is subjected to circuit formation and via hole formation to produce a multilayered printed circuit board.
The method in which a liquid resin is directly applied onto an inner circuit has problems that it is difficult to apply the resin with good thickness precision, and that polishing and other steps require much labor when a circuit is formed through plating. In the case of using a resin film, which is produced by applying a resin composition to a plastic film, there is a problem that the plastic film, which is discarded after use, is costly. Hence, the method in which a resin-coated copper foil is used is more common. As the resin ingredient, an epoxy resin is frequently used. Epoxy resins can fully satisfy the property requirements in general printed circuit boards because they are excellent in electrical insulating properties and chemical resistance. However, epoxy resins have limited heat resistance and, hence, there have been cases where they cannot be used as a material for printed circuit boards required to have high heat resistance.
The present inventors proposed the use of a resin composition comprising the following ingredients as a resin ingredient for a resin-coated copper foil (Japanese Patent Application No. 176565/1997).
(1) An epoxy resin and a hardener therefor, in a content of 40 to 80 parts by weight per 100 parts by weight of the total amount
(2) A maleimide compound in a content of 10 to 50 parts by weight per 100 parts by weight of the total amount
(3) A polyvinyl butyral resin having at least one polymerizable double bond as a functional group, in a content of 5 to 30 parts by weight per 100 parts by weight of the total amount
Due to the above makeup, not only the resin can have greatly improved heat resistance, but also the brittleness of the maleimide compound can be reduced.
However, the above resin composition has a problem that it has an increased coefficient of thermal expansion at high temperatures because of the use of a polyvinyl butyral resin. An increase in the coefficient of thermal expansion may cause problems such as cracking in a severe thermal cycle test, etc. and position shifting after parts mounting.
Accordingly, an object of the present invention is to provide a resin composition for copper-clad laminates which has a low coefficient of thermal expansion and high heat resistance and has extremely high crack resistance even upon undergoing a mechanical or thermal shock, thereby eliminating the above-described technical problems of prior art techniques, and to provide a resin-coated copper foil made by using the resin composition.
Another object of the present invention is to provide a multilayered copper-clad laminate and a multilayered printed circuit board both obtained using the resin composition for copper-clad laminates, which has such high heat resistance and high crack resistance, and using the resin-coated copper foil.